Blow molding is widely used to make hollow articles including bottles, jars, tanks (e.g. automobile gas tanks), and buoyant articles (e.g. floats).
In the blow molding process, molten polymer is extruded downward from a die, the extrudate forming a tube, called a parison. When the parison reaches the desired length, a two-piece mold closes, pinching off and sealing the bottom of the parison, and molding the top of the parison to form an article into the desired shape, such as the neck of the bottle. Gas is injected into the parison, expanding it to fill the mold, thereby giving the article its shape. The mold then opens and the article is separated from the mold, after which it may be subjected to further finishing, such as removal of mold flash. This cycle is repeated to create another article. The polymer used in blow molding must have sufficient melt strength so that the parison does not stretch or separate in the interval between the beginning of extrusion of the parison to the time when the parison has been expanded to fill the mold. Parison “hang time” is one method of determining if a polymer has sufficient melt strength for blow molding. The amount of time the formed parison can hang from the die without distortion is called “hang time”. While an acceptable hang time is at least 6 seconds, and preferably 10 seconds, “hang time” is dependent on the machine and/or the article being molded. Longer “hang times” have little benefit because of the interest manufacturers have in high productivity, and also because during hang time, the polymer cools and increases in viscosity. For semicrystalline polymer, cooling eventually leads to crystallization. Increased viscosity and crystallization both make the polymer less melt fabricable and adversely affect the expansion of the parison to fill the mold.
Reference is made to Irvin I. Rubin's, Handbook of Plastic Materials and Technology, New York, N.Y.: Wiley, 1990, page 1066 which discloses that resins (polymers) that do not have melt strength, such as injection molding grades, are difficult to blow mold. What usually occurs is that the hanging parison begins to stretch or draw down under its own weight. The top part of the parison begins to thin, which, in turn, will lead to poor material distribution and ultimately poor part performance.
Polymers such as PFA (perfluoroalkoxy copolymer, a copolymer of tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ether) (PAVE)) and FEP (fluorinated ethylene propylene), a copolymer of hexafluoropropylene (HFP) and tetrafluoroethylene (TFE), have found only limited use in blow molding applications, such as in making small articles. PFA that is sufficiently low in viscosity to be extruded at reasonable rates does not have enough melt strength. Therefore, the parison exhibits short hang time and tends to sag or even separate or tear when attempts are made to form larger articles, such as 1 quart (0.95 liters) or larger containers, and particularly, 1 gallon (3.79 liters) or larger containers. PFA that has melt strength adequate to provide acceptable hang times extrudes too slowly for economical production. Furthermore, slow extrusion of the PFA results in cooling of the lower end of the parison, so that by the time extrusion is complete, the lower end has higher viscosity than the upper end of the parison and may have begun to crystallize. Attempting to expand such a parison, results in distorted articles and even articles with holes. This leads to product defects and high scrap levels. Similarly, FEP is subject to the same limitations that occur with PFA. In summary, these perfluoropolymers are limited in their utility in blow molding because as molded articles increase in size, production rates decrease and/or losses due to quality defects increase.
It is thus desirable, to provide melt-fabricable perfluoropolymers that are usable in the making of larger articles of high quality from the blow molding process which can be extruded rapidly to give a parison of sufficient strength that does not sag, separate or tear, in the interval between extrusion and expansion in the closed mold better than that possible with commercial PFA, FEP or other perfluoropolymers.